A variable displacement compressor is used in air conditioning systems installed in vehicles for air-conditioning. Such type of compressor is disclosed in Japanese Unexamined Patent Publication No. 63-16177.
The publication discloses a compressor which is shown in FIG. 5 of the present application and to which attention should be directed. The compressor has a housing 71 in which a crank chamber 72 is formed. A drive shaft 73 is rotatably supported in the crank chamber 72. A rotor 74 is secured on the drive shaft 73, and a rotary swash plate 75 is rotatably and swingably supported on the drive shaft 73. The rotary swash plate 75 is coupled via a hinge mechanism 76 to the rotor 74. The hinge mechanism 76 consists of an elongated hole 74a provided in the rotor 74 and a pin 75a. The pin 75a is attached to the swash plate 75 and is engaged with the elongated hole 74a. The swash plate 75 is coupled to the rotor 74 and swingable within the range of the length of this elongated hole 74a. An undulation plate 77 is attached to the swash plate 55 with its rotation restricted.
A plurality of bores 78 are formed in the housing 71. A piston 79 is placed in each bore 78. The piston 79 is coupled to the undulation plate 77 and reciprocates within the corresponding bore 78 based on the undulation of the plate 77. A suction chamber 80 is formed adjacent to each bore 78 in the housing 71. A fluid (refrigerant) is supplied to each bore 78 from the suction chamber 80. Likewise, a discharge chamber 81 is formed adjacent to each bore 78 in the housing 71. The fluid compressed by the pistons 79 in the respective bores 78 is discharged into the discharge chamber 81. Formed in the housing 71 is a fluid passage 82 which communicates the crank chamber 72 with the suction chamber 80. Provided in the suction chamber 80 is valve means 83 which senses the pressure in the chamber 80 and adjusts the opening of the bleed fluid passage 82 in response to the pressure.
The thus constituted compressor functions as follows. As the valve means 83 operates in response to the suction pressure in the suction chamber 80, the opening of the bleed passage 82 is adjusted. At this time, the pressure in the crank chamber 72 varies from time to time due to the blow-by gas leaking from each bore 78. This pressure change alters the force acting on the back of the associated piston 79 and the balancing point of the moment that acts on the rotary swash plate 75, thus changing the inclination angle of the swash plate 75 and the undulation plate 77. The stroke of each piston 79 changes due to the angular change, so that the compression displacement of the fluid in each bore 78 is changed, controlling the amount of the fluid led into the bore 78. The suction pressure in the suction chamber 80 is controlled so as to be a predetermined value by the mechanism which varies the compression displacement in this manner.
According to the aforementioned variable displacement type mechanism, as the suction pressure in the suction chamber 80 falls due to a decrease in the thermal load in the air conditioning system, the valve means 83 is operated to reduce the opening of the fluid passage 82. The pressure increase in the crank chamber 72 is accelerated to control the compression displacement of the compressor in the direction of reducing it. When the thermal load further decreases, the valve means 83 is operated to completely close the fluid passage 82, so that the pressure in the crank chamber 72 further rises. This further reduces the compression displacement.
Even in this case, the reduction of the compression displacement is restricted to a predetermined minimum value. This is because that in an extremely small displacement area where the compression displacement becomes zero or extremely small close to zero, no effective compression-oriented work is performed. The restoration of the compression displacement, which should be accomplished by the difference between the suction pressure in the suction chamber 80 and the pressure in the crank chamber 72, becomes thus practically impossible. In recent compressors in which the individual sliding portions in the compressors are required to be lubricated with the oil mist admixed in the refrigerant, the burning at the individual sliding portions and the reduction in durability of the compressors due to insufficiency of refrigerant (lubrication) can be pointed as the factors which restrict the minimum value of the variable compression displacement.
So long as the minimum compression displacement is restricted as mentioned above, when a vehicular air conditioning system including a compressor and evaporator is used in the environment of a cold place or the like, the operation of the compressor must be controlled properly to protect the sliding portions of the compressor against wear and prevent freezing of the evaporator. For example, the compressor should be properly stopped by cutting off the power transmission to the compressor by an electromagnetic clutch. The electromagnetic clutch coupled to the compressor is widely used as an essential component of the current vehicular air conditioning systems.
With the use of a vehicular air conditioning system that employs an electromagnetic clutch, however, the shock at the time the system is activated thereby affects the driving feeling of the vehicle. In addition, the alternator for supplying power to the electromagnetic clutch has a surprisingly low efficiency, increasing the engine load accordingly, which is not negligible, either. In other words, it is easily understood that the elimination of the electromagnetic clutch, if possible, can significantly reduce the weight of the vehicular air conditioning systems as well as can contribute to reducing the fuel consumption.
It is therefore an object of the present invention to ensure protection of the individual sliding portions of a compressor against wear and suppression of over-cooling or the like, while eliminating the electromagnetic clutch.